[PATCH] drm: Don't zero vblank timestamps from the irq handler

[Thierry Reding]

On Wed, Sep 30, 2015 at 07:21:34PM +0300, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> If we couldn't get a high precisions vblank timestamp, we currently
> store a zeroed timestamp instead and assume the next vblank irq to
> get us something better. This makes sense when trying to update the
> timestamp from eg. vblank enable. But if we do this from the vblank
> irq we will never get a vblank timestamp unless we high precision
> timestamps are available and succeeded. This break weston for instance
> on drivers lacking high precision timestamps.
> 
> To fix this, zero the timestamp only when not called from vbl irq.
> When called from the irq, we still want the timestamp, even if not
> perfect.
> 
> This fixes a regression from
> 4dfd64862ff852df drm: Use vblank timestamps to guesstimate how many vblanks were missed
> 
> Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
> Cc: Thierry Reding <thierry.reding at gmail.com>
> Reported-by: Thierry Reding <thierry.reding at gmail.com>
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/drm_irq.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)

Applied on top of next-20151001 and the weston problem I was seeing is
gone, so:

Tested-by: Thierry Reding <treding at nvidia.com>

I think it might be worth considering squashing this into the offending
commit to avoid breaking bisectibility.

Thierry
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